1. Field of the Invention
The present invention relates to a builtup battery consisting of a plurality of non-soluble type of secondary batteries, such as lithium ion secondary batteries, and more particularly, it relates to a series connection circuit for the secondary batteries, which enables to prevent the builtup battery including the non-soluble type of secondary batteries connected in series with each other from overcharging and overdischarging electricity.
2. Description of Related Art
In Japanese Unexamined Patent Publication (Kokai) Nos. 4-331425 and 61-206179, there are disclosed respective devices which prevent the non-soluble type of secondary batteries such as lithium ion batteries from overcharging and overdischarging the electricity. FIG. 1 shows a construction of the conventional device for preventing a builtup battery consisting of two lithium ion secondary batteries 21a and 21b connected with each other in series from overcharging and overdischarging.
Generally speaking, although the lithium ion battery has advantages of less self-discharge and higher charging efficiency, it is characterized by a "designed" voltage which is determined by a chemical reaction between active materials of anode and cathode and organic electrolyte and which is also called "the charge stop voltage". Therefore, in general, to charge the batteries in excess of the charge stop voltage is called the overcharging. Under this overcharging condition, it causes troubles of a precipitation of metallic lithium on the cathode, a precipitation of metallic cobalt or cobalt compounds precipitated due to a cobalt ion which may be produced by dissolution of the anodic active materials and a dissolution of the organic electrolyte. Consequently, the above troubles causes a problem of short circuit between the anode and the cathode and a deterioration of battery in performance. Therefore, from these points of view, it is necessary to exclude this overcharging problem from the builtup battery.
Similarly, to discharge the battery till below a dissoluble voltage (or "the discharge stop voltage" ) of electrode materials (e.g. Cu) is called the overdischarging. Under this overdischarging condition, since the ionized cupper (Cu) is dissolved to flow into the electrolyte, the current collecting function is so lowered that the active substances of the electrodes fall off. Consequently, although the extent of damage is somewhat small in comparison with that in the "overcharging" case, the overdischarging also causes a trouble of short circuit between the anode and the cathode due to the cupper ion dissolving into the electrolyte and a problem of reduction in the battery capacity.
In order to avoid the above overcharging and overdischarging problems, in the conventional device shown in FIG. 1, when charging the lithium ion secondary batteries 21a and 21b upon connecting a not-shown battery for charge with a common terminal 30 and a charging terminal 31 of the device and further switching on a switch 25, two voltage detectors 24a and 24b detect voltage-values of the batteries 21a and 21b, respectively. Then, as soon as either one of the detected values is in excess of a predetermined value, a switch 23 constituted by FET (Field Effect Transistor), SSR (Solid State Relay) or the like is switched off, whereby the current for charging the batteries is cut off to avoid the overcharging.
On the other hand, in case of discharging, upon switching on the switch 25, the discharging voltage value is checked by a voltage detector 26. Then, as soon as the detected voltage value is less than the dissoluble voltage of electrode materials, the switch 25 is switched off, whereby the overdischarging for the batteries 21a and 21b is avoided.
In the above-mentioned conventional device as shown in FIG. 1, however, the device is only constructed in such a manner that when the total voltage at discharging is less than the dissoluble voltage, then the discharging operation is stopped and when any one of the batteries contained therein amounts to the predetermined full voltage level during charging, then the charging operation is stopped. Therefore, the device has no function of restricting an increasing of a difference of "DOD" (Depth of discharge) values(%) of the batteries 21a and 21b. Note, in general, the difference of DOD values is caused by a difference of charging/discharging efficiencies between the respective batteries. The difference of charging/discharging efficiencies is derived from an unequal temperature distribution etc.
We describe the difference of DOD in detail, with reference to FIG. 3 where one changing form of charging/discharging efficiencies is illustrated. When two lithium batteries A and B are used, connected in series with each other, the available battery capacity is restricted by the smaller one in battery capacity of the batteries A and B. Therefore, provided that both the battery A of a battery capacity of 100 Ah (Ampere/hour) at the first charging and L the battery B of a battery capacity of 99 Ah, of which difference is derived from the difference of DOD, are discharged together, there would be remained a uselessness of 1 Ah since the respective energy consumption are 99 Ah in both batteries A and B, as shown in a left part of FIG. 2. Similarly, after the tenth discharging operation has been finished, the uselessness of battery capacity in the battery A amounts to 9.5 Ah. In this way, corresponding to the increasing of the difference of DOD, a problem that total amounts of energy usable as the builtup battery is gradually decreased is raised.
Furthermore, the lithium ion secondary battery is characterized by a large deterioration rate of battery-capacity caused by overcharging. FIG. 3 shows an example of the change of deterioration rate of battery-capacity corresponding to numbers of overcharging cycle. It will be easily understood by the figure that, under condition that the charging is carried out with a constant current/constant voltage, a change of setting voltage from 4.2 V to 4.3 V, i.e., the changes by only 0.1 V, causes an abrupt increasing of the deterioration rate of battery-capacity from 2.5% to, approx. 10% at the discharging/charging of 100 cycles. This means that a life span of the battery is lowered to a quarter by a difference of 0.1 V (ratio: 2.4% for 4.2 V) in set voltage and that the life span is remarkably shortened by the increase of difference of DOD.
Note that, in comparison with the deterioration of battery-capacity by overcharging, the lithium ion secondary battery has a small deterioration rate of battery-capacity by overdischarging. FIG. 4 shows an example of the change of deterioration rate of battery-capacity corresponding to numbers of overdischarging cycle. It will be easily understood by the figure that, even if the discharge stop voltage (dissoluble voltage) is changed from 2.4 to 1.0 V, the deterioration rate of battery-capacity changes by an increasing of from 2.5% to 6%. This means that the life span of the battery is lowered to only a half in spite of a difference of 1.5 V (ratio: 60% for 2.5V) in discharge stop voltage.
FIG. 5 shows another conventional series connection circuit. The circuit includes a plurality of lithium secondary batteries 41a, 41b, . . . and 41n which are connected in series with each other. In the batteries, metallic lithium is employed for the cathodes and on the other hand, either activated carbon or titanium disulfide/manganese oxide is used for the anodes. In this circuit, in order to avoid a deterioration of the battery performance due to differences in charging character among the respective batteries, which is caused by the uneven distribution of charging-voltage to the batteries, constant voltage circuits (Zener Diode etc.) ZDa, ZDb, . . . and ZDn are connected in parallel with the batteries, respectively, to detect voltage values applied on the respective batteries.
With the above-mentioned arrangement, however, since the voltage values are detected by the constant voltage circuits connected in parallel with the respective batteries 41a, 41b, . . . and 41n, it is inevitable that, due to characteristic variance in and around the Zener diodes, a difference in voltage detecting character (level) among the respective batteries is produced.
Referring to FIG. 6, we now suppose that a detected voltage value of a battery E is lower than the actual voltage thereof by 0.1 V (actual voltage at full-charging: 4.3 V); the detected voltage of a battery F is detected equal to the actual voltage (actual voltage at full-charging: 4.2 V); and that a detected voltage value of a battery G is higher than the actual voltage thereof by 0.1 V (actual voltage at full-charging: 4.1 V). Under the above condition, in case of using the full-charged lithium secondary batteries E (battery capacity: 110Ah), F (battery capacity: 100Ah) and G(battery capacity: 90Ah) as the series connection builtup battery, the usable battery capacity is limited to the smallest capacity of the three, i.e., 90Ah of the battery G, so that only 90% of battery capacity is effected in the battery F. Again, in this case, since the battery E is under the overcharging condition, the life span thereof is shortened at first of the three. In conclusion, the above-mentioned circuit has a problem of reduction of energy usable for the builtup battery due to the difference of voltage detection, whereby the life span of the battery is shortened.